Nicomachean Ethics

Aristotle asks what the best life for a human being is, and to answer it he examines what virtue is and which virtues make up such a life. Divided into ten books, the work treats ethics as practical rather than theoretical: its aim is to help people become good, not only to define the good. It pairs with his Politics, which pursues a similar goal for a whole community, and three of its books are identical to books of his Eudemian Ethics. Thomas Aquinas referred to Aristotle simply as the Philosopher, and the work remained widely accepted as the main authority on ethics at Protestant universities into the 17th century. The modern virtue ethics revival renewed interest in it.

Ryan Holiday reread it and said he could not recommend it highly enough. Mark Manson describes it as the root of Aristotelian ethics and of many moral ideas underlying western civilization.